In a semiconductor material, band gap is an important parameter, which to a large extent determines properties of the semiconductor material. The band gap is defined as the difference in energy between the top of the valence band and the bottom of the conduction band. This is the energy that is required to excite an electron from the valence band into the conduction band. Electrons in the conduction band have the ability to move through the material, thereby enabling conduction of electricity.
One type of semiconductor material, graphene, is of great interest for nanoscale electronics due to the much higher carrier mobility compared to Silicon. Graphene is a two-dimensional planar sheet of carbon atoms arranged in a hexagonal benzene-ring structure. A free-standing graphene structure is theoretically stable only in a two-dimensional space, which implies that a planar graphene structure does not exist in a free state, being unstable with respect to formation of curved structures such as soot, fullerenes, and nanotubes. However, a two-dimensional graphene structure has been demonstrated on a surface of a three-dimensional structure, for example, on the surface of a Silicon Dioxide (SiO2). A typical graphene layer may comprise a single sheet or multiple sheets of carbon atoms, for example, between 1 sheet and 10 sheets.
Field-effect transistor (FET) is a dominant and important device in fabricating integrated circuits. FET may be used for amplifying, switching, and detecting signals. In a FET device, the FET relies on an electric field to control the carrier density and hence the conductivity of a channel of one type of charge carrier. It is known that graphene has been used in forming a FET. Unfortunately, despite its high carrier mobility, graphene has a zero band gap, which leads to a very poor FET leakage current. One solution to this problem has been to use bi-layer graphene with both the top gate and the substrate to thereby open the band gap of the material. However, the substrate structure makes large-scale complementary-metal-oxide-semiconductor (CMOS) transistor impractical due to the lack of threshold voltage (Vt) control of an individual device.